This page intentionally left blank LIGHT-EMITTING DIODES SECOND EDITION Revised and fully updated, the Second Edition of this textbook offers a comprehensive explanation of the technology and physics of light-emitting diodes (LEDs) such as infrared, visible-spectrum, ultraviolet, and white LEDs made from III–V semiconductors. The elementary properties of LEDs such as electrical and optical characteristics are reviewed, followed by the analysis of advanced device structures. With nine additional chapters, the treatment of LEDs has been vastly expanded, including new material on device packaging, reflectors, UV LEDs, III–V nitride materials, solid-state sources for illumination applications, and junction temperature. Radiative and non-radiative recombination dynamics, methods for improving light extraction, high-efficiency and high- power device designs, white-light emitters with wavelength-converting phosphor materials, optical reflectors, and spontaneous recombination in resonant-cavity structures, are dis- cussed in detail. Fields related to solid-state lighting such as human vision, photometry, colorimetry, and color rendering are covered beyond the introductory level provided in the first edition. The applications of infrared and visible-spectrum LEDs in silica fiber, plas- tic fiber, and free-space communication are also discussed. Semiconductor material data, device design data, and analytic formulae governing LED operation are provided. With exercises, solutions and illustrative examples, this textbook will be of interest to scientists and engineers working on LEDs, and to graduate students in electrical engineering, applied physics, and materials science. Additional resources for this title are available online at www.cambridge.org/ 9780521865388. E. Fred Schubert received his Ph.D. degree with Honors in Electrical Engineering from University of Stuttgart in 1986 and is currently a Wellfleet Senior Constellation Pro- fessor of the Future Chips Constellation at Rensselaer Polytechnic Institute. He has made several pioneering contributions to the field of LEDs, including the first demonstration of the resonant-cavity light-emitting diode (RCLED). He has authored or co-authored more than 200 publications including Doping in III–V Semiconductors (Cambridge University Press, 1993, 0-521-01784-X) for which he was awarded the VDE Literature Prize. He is inventor or co-inventor of 28 US Patents and a Fellow of the IEEE, APS, OSA, and SPIE. He received the Senior Research Award of the Humboldt Foundation, the Discover Award for Technological Innovation, the RD 100 Award, and Boston University’s Provost Innovation Fund Award. Note: This book contains many figures in which color adds important information. For this reason, all figures are available in color on the Internet at the following websites: < http://www.cambridge.org/ 9780521865388> and < http://www.LightEmittingDiodes.org >. LIGHT-EMITTING DIODES SECOND EDITION E. FRED SCHUBERT Rensselaer Polytechnic Institute, Troy, New York CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521865388 © First edition E. Fred Schubert 2003 Second edition E. Fred Schubert 2006 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2006 ISBN-13 978-0-511-34476-3 eBook (EBL) ISBN-10 0-511-34476-7 eBook (EBL) ISBN-13 978-0-521-86538-8 hardback ISBN-10 0-521-86538-7 hardback Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Contents Preface page x 1 History of light-emitting diodes 1 1.1 History of SiC LEDs 1 1.2 History of GaAs and AlGaAs infrared and red LEDs 4 1.3 History of GaAsP LEDs 8 1.4 History of GaP and GaAsP LEDs doped with optically active impurities 9 1.5 History of GaN metal−semiconductor emitters 15 1.6 History of blue, green, and white LEDs based on GaInN p-n junctions 17 1.7 History of AlGaInP visible-spectrum LEDs 19 1.8 LEDs entering new fields of applications 21 References 23 2 Radiative and non-radiative recombination 27 2.1 Radiative electron−hole recombination 27 2.2 Radiative recombination for low-level excitation 28 2.3 Radiative recombination for high-level excitation 32 2.4 Bimolecular rate equations for quantum well structures 33 2.5 Luminescence decay 33 2.6 Non-radiative recombination in the bulk 35 2.7 Non-radiative recombination at surfaces 41 2.8 Competition between radiative and non-radiative recombination 44 References 46 3 Theory of radiative recombination 48 3.1 Quantum mechanical model of recombination 48 3.2 The van Roosbroeck–Shockley model 50 3.3 Temperature and doping dependence of recombination 54 3.4 The Einstein model 56 References 57 4 LED basics: Electrical properties 59 4.1 Diode current–voltage characteristic 59 4.2 Deviations from ideal I–V characteristic 63 4.3 Evaluation of diode parasitic resistances 67 4.4 Emission energy 68 4.5 Carrier distribution in p-n homojunctions 69 4.6 Carrier distribution in p-n heterojunctions 70 4.7 Effect of heterojunctions on device resistance 71 4.8 Carrier loss in double heterostructures 75 4.9 Carrier overflow in double heterostructures 78 4.10 Electron-blocking layers 81 4.11 Diode voltage 83 References 84 5 LED basics: Optical properties 86 5.1 Internal, extraction, external, and power efficiencies 86 5.2 Emission spectrum 87 v 5.3 The light escape cone 91 5.4 Radiation pattern 93 5.5 The lambertian emission pattern 94 5.6 Epoxy encapsulants 97 5.7 Temperature dependence of emission intensity 98 References 100 6 Junction and carrier temperatures 101 6.1 Carrier temperature and high-energy slope of spectrum 101 6.2 Junction temperature and peak emission wavelength 103 6.3 Theory of temperature dependence of diode forward voltage 104 6.4 Measurement of junction temperature using forward voltage 108 6.5 Constant-current and constant-voltage DC drive circuits 110 References 112 7 High internal efficiency designs 113 7.1 Double heterostructures 113 7.2 Doping of active region 116 7.3 p-n junction displacement 118 7.4 Doping of the confinement regions 119 7.5 Non-radiative recombination 122 7.6 Lattice matching 123 References 126 8 Design of current flow 127 8.1 Current-spreading layer 127 8.2 Theory of current spreading 133 8.3 Current crowding in LEDs on insulating substrates 136 8.4 Lateral injection schemes 140 8.5 Current-blocking layers 142 References 143 9 High extraction efficiency structures 145 9.1 Absorption of below-bandgap light in semiconductors 145 9.2 Double heterostructures 149 9.3 Shaping of LED dies 150 9.4 Textured semiconductor surfaces 154 9.5 Cross-shaped contacts and other contact geometries 156 9.6 Transparent substrate technology 157 9.7 Anti-reflection optical coatings 159 9.8 Flip-chip packaging 160 References 161 10 Reflectors 163 10.1 Metallic reflectors, reflective contacts, and transparent contacts 164 10.2 Total internal reflectors 168 10.3 Distributed Bragg reflectors 170 10.4 Omnidirectional reflectors 181 10.5 Specular and diffuse reflectors 184 References 189 vi 11 Packaging 191 11.1 Low-power and high-power packages 191 11.2 Protection against electrostatic discharge (ESD) 193 11.3 Thermal resistance of packages 195 11.4 Chemistry of encapsulants 196 11.5 Advanced encapsulant structures 198 References 199 12 Visible-spectrum LEDs 201 12.1 The GaAsP, GaP, GaAsP:N, and GaP:N material systems 201 12.2 The AlGaAs/GaAs material system 206 12.3 The AlGaInP/GaAs material system 209 12.4 The GaInN material system 211 12.5 General characteristics of high-brightness LEDs 213 12.6 Optical characteristics of high-brightness LEDs 216 12.7 Electrical characteristics of high-brightness LEDs 218 References 220 13 The AlGaInN material system and ultraviolet emitters 222 13.1 The UV spectral range 222 13.2 The AlGaInN bandgap 223 13.3 Polarization effects in III–V nitrides 224 13.4 Doping activation in III–V nitrides 226 13.5 Dislocations in III–V nitrides 227 13.6 UV devices emitting at wavelengths longer than 360 nm 231 13.7 UV devices emitting at wavelengths shorter than 360 nm 233 References 236 14 Spontaneous emission from resonant cavities 239 14.1 Modification of spontaneous emission 239 14.2 Fabry−Perot resonators 241 14.3 Optical mode density in a one-dimensional resonator 244 14.4 Spectral emission enhancement 248 14.5 Integrated emission enhancement 249 14.6 Experimental emission enhancement and angular dependence 251 References 253 15 Resonant-cavity light-emitting diodes 255 15.1 Introduction and history 255 15.2 RCLED design rules 256 15.3 GaInAs/GaAs RCLEDs emitting at 930 nm 260 15.4 AlGaInP/GaAs RCLEDs emitting at 650 nm 265 15.5 Large-area photon recycling LEDs 268 15.6 Thresholdless lasers 270 15.7 Other RCLED devices 271 15.8 Other novel confined-photon emitters 272 References 273 16 Human eye sensitivity and photometric qualities 275 16.1 Light receptors of the human eye 275 vii 16.2 Basic radiometric and photometric units 277 16.3 Eye sensitivity function 280 16.4 Colors of near-monochromatic